1. Field of the Invention
The present invention relates to a nitride semiconductor device using a nitride semiconductor and a method for manufacturing the same.
2. Description of Related Art
A semiconductor light emitting device using a nitride semiconductor has already been practically used as a light emitting diode in various kinds of fields because it is able to emit light over wide wavelength ranges from ultraviolet to blue or green visible light by suitably controlling the composition of the nitride semiconductor.
As the nitride semiconductor has a large band gap and a deep acceptor level, it is difficult to reduce the resistance of a p-type layer. However, in order to reduce the operating voltage of a light emitting diode having a p-n junction, it is essential to reduce the resistance of the p-type layer and contact resistance between the p-type layer and an ohmic electrode.
Hereinafter, as a first conventional example, a light emitting diode using a nitride semiconductor is explained with reference to drawings (see, for example, Shuji Nakamura et al., “Superbright green InGaN single-quantum-well-structure light-emitting diodes”, Jpn. J. Appl. Phys. Vol. 34 (1995) L.1332-L.1335).
FIG. 14 shows the sectional structure of the first conventional example of the light emitting diode using a nitride semiconductor. As shown in FIG. 14, the conventional light emitting diode includes a sapphire substrate 101 having a (0001) plane as a principal surface. On the principal surface of the sapphire substrate 101, a GaN buffer layer 102 which grows at low temperatures, an n-type GaN layer 103, an InGaN quantum well active layer 104, a p-type cladding layer 105 made of p-type AlGaN and a p-type contact layer 106 made of p-type GaN are formed in this order. A p-type electrode 107 is formed on the p-type contact layer 106 and an n-type electrode 108 is formed on a selectively exposed part of the n-type GaN layer 103.
In the first conventional example, the p-type cladding layer 105 contains magnesium (Mg) as p-type dopants and heat treatment is carried out in nitrogen atmosphere to obtain a p-type semiconductor layer.
As a second conventional example of a semiconductor device using the nitride semiconductor, a blue-violet semiconductor laser device has already come into practical use. Commonly used blue-violet semiconductor laser devices have a ridge waveguide structure as disclosed by Shuji Nakamura et al., “InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitalixally laterally overgrown GaN substrate”, Appl. Phys. Lett. Vol. 72 (1998) pp. 211-213. In this structure, a p-type semiconductor layer on an active layer is patterned into a convex stripe in order to narrow a current flow and trap emitted light. The blue-violet semiconductor laser device is achieved by a relatively easy process.
However, the light emitting diode using the nitride semiconductor of the first conventional example has limitations in reducing the resistance of the p-type contact layer 106 and the contact resistance between the p-type contact layer 106 and the p-type electrode 107. Therefore, it is difficult to reduce the operating voltage to a further extent. Likewise, it is not easy to reduce the resistance of the p-type contact layer in the semiconductor laser device using the nitride semiconductor. Thus, difficulty remains in improving characteristics, efficiency and reliability of the laser device.
In the semiconductor laser device using the nitride semiconductor of the second conventional example, the ridge waveguide structure having a convex section is provided by dry etching. Therefore, it is difficult to control the thickness of parts of the p-type semiconductor layer remaining on the sides of the ridge. This leads to variations in threshold current in the semiconductor laser device and variations in angle of light emission.
Moreover, since the p-type semiconductor layer is made of a nitride semiconductor which is not suitable for easily producing a low resistance p-type semiconductor layer, the series resistance of the p-type semiconductor layer is increased. Further, electrooptic characteristics and reliability of the laser device may possibly deteriorate due to damage caused by the dry etching for forming the ridge. Thus, the semiconductor laser device of the second conventional example has limitations in improving yield and reducing the operating voltage.